It is common to detect a target object from a medical image in the medical image processing field. For example, it is of important clinical significance to determine a sagittal plane image (also called a median sagittal plane image) in which a spine is located from a series of sagittal plane images of a human body, and to detect intervertebral disks from the determined sagittal plane image. Information of the detected intervertebral disks, such as the position and the direction thereof, can be used to guide the subsequent spine scanning performed in an image of higher quality.